高井 健一

Hydrogen and deuterium trapping sites in high-strength steels have been observed by secondary ion mass spectrometry(SIMS). High-strength steels with 1400 MPa tensile strength are stressed and dipped in D2O and 20% NH4SCN solution at 323 K to occlude hydrogen and deuterium. Depth profiles by SIMS show the presence of deuterium, which indicates hydrogen trapping sites occluded during the delayed fracture test. Secondary ion image analysis by SIMS has made it possible to observe hydrogen and deuterium trapping sites in high-strength steels. Hydrogen tends to accumulate at grain boundaries, in segregation bands, and on inclusions. Line scans by SIMS have shown that the accumulation ratios of hydrogen for the grain boundaries, segregation bands of P and inclusions are 7.8, 5.0 and 11.0 times higher than that in the matrix, respectively. Hydrogen trapping sites at the grain boundaries can be observed by measuring within 24 h after the delayed fracture test.

Effects of Si and Ca addition on delayed fracture of medium carbon steels with 1400N/mm^2 strength were investigated. Silicon and calcium were added at concentrations of 0∿2.0% and 30∿70ppm, respectively. The delayed fracture characteristics were evaluated by FIP (Federation Internationale de la Precontrainte) test which is a constant tensile load test in 20% NH_4SCN solution at 323 K. In order to make clear the effect of adding Ca and Si, the fracture surfaces were examined, and the hydrogen evolution behavior, the diffusion coefficient of hydrogen, and the hydrogen content were measured. It was found that 0.5%Si steels have no effect on the time to fracture regardless of Ca content, while 1.5%Si-30ppm Ca steel has the longest time to fracture. Fractography showed that adding Ca to 0.5%Si steels did not change the intergranular fracture area fraction. However, adding Ca to 1.5%Si steels changed the fracture from intergranular fracture to microvoid coalescence fracture. As for hydrogen behavior after three months from FIP test, 0.5%Si steel released hydrogen at the peak of 500 K, while for the 1.5%Si-30ppm Ca steel the peak was at 700 K. It was suggested that hydrogen released at around 500 K was crucial for delayed fracture characteristics.